1. Field of the Invention
The present invention relates to quantum cascade lasers, and quantum cascade lasers for broadband applications.
2. Description of the Related Art
Over recent decades, several sources with mid-infrared (IR) emission have been developed, including thermal IR sources, optical parametric oscillators (OPO), light emitting diodes (LED), cryogenically cooled lead-salt diode lasers (pb-salt), and quantum cascade (QC) semiconductor lasers. QC laser technology has been successfully demonstrated in the 4 to 150 μm wavelength range and is the only technology demonstrated to provide significant optical power over the entire wavelength range of interest. Thus, QC lasers have become the most promising sources in the mid- to far-infrared range. Compared with typical thermal IR sources, the wavelength coverage of a typical QC laser is inadequate due to its narrow waveband. In addition, such lasers are often too bulky, complex and are based on technology too unreliable for space applications. Thus, some other approach is needed to yield a miniaturized, compact, robust, and reliable broadband laser source in the mid-IR wavelength region.
Quantum cascade lasers are semiconductor devices that emit electromagnetic radiation in the mid-to far infrared frequency spectrum, with numerous applications, such as for example chemical monitoring, medical diagnostics, collision avoidance using lidar, and free space communication, to name just a few. Quantum cascade lasers are unipolar devices, where a single type of carrier, usually electrons, emit photons when transitioning from an energy band to a lower energy band. Energy bands are engineered with the use of quantum wells. A quantum cascade laser comprises a number of active regions, each active region including an injector region adjacent to a quantum well. Electrons tunnel through an injector region so as to be injected into an adjacent quantum well. The energy bands are structured such that an electron injected into a quantum well emits a photon when transitioning from an energy band to a lower energy band within that quantum well, where the electron then tunnels through the next injector to the next quantum well, where it again may transition from an energy band to a lower energy band within that next quantum well to emit another photon. This cascading process continues, and is one of the reasons why quantum cascade lasers are efficient sources of laser radiation.
For some applications, it is desirable to have a tunable broadband laser source. For example, a tunable broadband source may be of utility in probing gases for their chemical makeup, where the spectral content of the probing signal gives information about the chemical species, or may be of utility in a communication system, to name a couple of examples.
FIG. 1 illustrates in a simplified pictorial cross-sectional view a prior art quantum cascade laser for providing broadband radiation. In between cladding layers 102 and 104 are two active regions, each providing radiation at a different wavelength. For ease of illustration, only two active regions are illustrated in FIG. 1, active region 106 to provide radiation having a first wavelength (λ1) and active region 108 to provide radiation having a second wavelength (λ2). In practice, however, there may many active regions, each one providing electromagnetic radiation at a different wavelength. The index of refraction of cladding layers 102 and 104 are less than that of the active regions, so that the structure of layers 102, 104, 106, and 108 form a ridge waveguide. In the particular example of FIG. 1, a voltage potential is provided between metal layer 110 and substrate layer 112, and the electromagnetic propagation is along the z-axis direction as indicated by the XYZ coordinate system illustrated in FIG. 1.
Each active region in FIG. 1 includes an injector region with an adjacent quantum well. A quantum well may be referred to as gain region. The injector region usually is a superlattice. The layers making up the superlattice injector regions and the quantum wells are formed along the y-axis direction by various well-known techniques, such as molecular beam epitaxy. By including many active regions, each emitting electromagnetic radiation at a different wavelength, a broadband laser source may be synthesized. However, a problem with quantum cascade lasers of the type depicted in FIG. 1 is that it may be difficult to control the individual active regions. For example, some active regions may provide more power than other active regions, and it may be difficult to individually tune the active regions so as to provide a desired spectral laser output.
In view of the foregoing, there is a need in the art for apparatuses and methods for tunable broadband mid-infrared sources. e.g. employing quantum cascade lasers. Particularly, there is a need for such apparatuses and method to afford higher optical power, spectral density and brightness in comparison to the typical thermal IR sources in the 3 to 20 micron wavelength region. Particularly, there is a need for such apparatuses and methods to exhibit sufficient reliability and robustness for space applications. These and other needs are met by embodiments of the present invention as detailed hereafter.